(1) Field of the Invention
The present invention is directed to helical antennas. In particular, the present invention is directed to a direct fed bifilar helix antenna that is broadband with low characteristic impedance.
(2) Description of the Prior Art
There exists in the prior art a family of broadband quadrifilar helix antennas such as the antennas described in U.S. Pat. No. 6,246,379 (Josypenko), that have the characteristics of being broadband above a cut-in frequency, and having a low voltage standing wave ratio (VSWR) above a cut-infrequency about the characteristic impedance (Z0) value of the antenna. Wide element quadrifilar helix antennas reduce the value of Z0 to a practical lower limit of Z0=100 ohms, which can then feed into the Z0=100 ohms between the two center conductors of a one hundred eighty degree power splitter feeding a given bifilar. The wide element quadrifilar helix antenna taught in U.S. Pat. No. 6,246,379 (Josypenko), comprises two crossed bifilar helixes and a 50 ohm ninety degree power splitter feeding two 50 ohm one hundred eighty degree power splitters feeding their two 100 ohm outputs directly into the two crossed bifilar helixes making up the quadrifilar helix. The wide element quadrifilar helix antenna does not require a matching network. The antenna is directly fed via its power splitter feed network.
The broadband impedance properties exhibited by wide element quadrifilar helix antennas also apply to bifilar helixes, since the quadrifilar helix is an array of two crossed bifilar helixes. The bifilar helix is the basic building block of the quadrifilar helix. A difference in the characteristic impedance Z0 between a wide element quadrifilar helix antenna (i.e., two crossed bifilars) and a wide element bifilar helix antenna is that when changing from two crossed bifilars to one, with the width of a bifilar element being the combined widths of the two quadrifilar elements it replaces, then the characteristic impedance is halved.
The halving of the characteristic impedance Z0 is explained as follows with accompanying FIGS. 1a and 1b. Z0 is calculated according to
            Z      0        =                  L        C              ,where L is the series inductance per unit length of the helix and C is the shunt capacitance per unit length of the helix. FIG. 1a shows a section 7 of quadrifilar helix unpitched the sources of capacitance per unit length of helix C along the helix length. The section is composed of sections 1, 2, 3 and 4 of the 4 elements of the helix of length 6 that is ⅛ wavelength or less, separated by small gaps G12, G23, G34, G41, all centered about helix axis 5. The capacitance between radially opposite elements 1 and 3 is shown as C13 between midpoints M1 and M3 of the element sections, capacitance C24 exists between midpoints M2 and M4 of element sections 2 and 4. Capacitance also exists between the elements at their gaps as C12, C23, C34 and C41 between element sections 1 and 2, 2 and 3, 3 and 4, and 4 and 1. Since the elements are much closer together at their gaps, the inter-gap capacitances are much larger than the radial capacitances. Thus, when finding the total capacitance between the midpoints of two radially opposite element sections, the radial capacitances C13 and C24 can be ignored. Thus, the capacitance between element sections 1 and 3 is the series capacitance of C12 and C23 in parallel with the series capacitance of C41 and C34, or:
      C    Total    =            (              1                              1                          C              ⁢                                                          ⁢              12                                +                      1                          C              ⁢                                                          ⁢              23                                          )        +          (              1                              1                          C              ⁢                                                          ⁢              41                                +                      1                          C              ⁢                                                          ⁢              34                                          )      with C12=C23=C34=C41=C from symmetry,
      C    Total    =            2                        1          C                +                  1          C                      =                  2                  2          C                    =              C        .            This is the capacitance per unit length between either pair of radially opposite elements. When the quadrifilar helix is changed to a bifilar helix, gaps G12 and G34, for example, are removed so that elements 1 and 2 combine to become the first element of the bifilar and elements 3 and 4 combine to become the second element of the bifilar. C12 and C34 are shorted out and disappear, so now the capacitance between only two element sections at new midpoints M1M2, and M3M4 becomes: CTotal=C23+C41=2C.
FIG. 1b shows the quadrifilar elements E1, E2, E3 and E4 unwrapped and unpitched to more easily show the inductance per unit length L1, L2, L3 and L4 of the elements in section 7. Due to symmetry, L1=L2=L3=L4=L. When the quadrifilar helix is changed to a bifilar case, element sections 1 and 2, for example, combine to a first bifilar element and element sections 3 and 4 combine to a second bifilar element. Gaps G12 and G34 are filled and disappear, and now the ends of L1 and L2 are considered connected with virtual connections C121 and C122; ends of L3 and L4 are considered connected with virtual connections C341 and C342. The inductance per unit length becomes the parallel combination of L1 and L2, or L3 and L4, or the inductance per unit length is
  =            1                        1                      L            ⁢                                                  ⁢            1                          +                  1                      L            ⁢                                                  ⁢            2                                =                  1                  2          L                    =                        L          2                .            
The characteristic impedance Z0 for a loss less transmission line is found by
      Z    0    =                    L        C              .  For the quadrifilar helix
      Z    0    =                    L        C              .  When the quadrifilar helix is converted to a bifilar helix, Z0 becomes:
      Z    0    =                              L          2                          2          ⁢                                          ⁢          C                      =                  1        2            ⁢                                    L            C                          .            
Thus Z0 is halved when the quadrifilar helix becomes a bifilar helix. Note this is an approximation for the case when the gap width is small. An even more precise value of Z0 can be obtained by adjusting the gap width, even if necessary to the point of negative gap width values, in which case the element edges overlap but do not touch.
A prior art bifilar helix antenna made of moderate width elements and a diameter of nine inches was investigated. To match the high characteristic impedance (Z0) elements of the helix to 50 ohms, a quarter wavelength transmission line transformer is connected at the feed point of the bifilar antenna. The outer conductor connects to the feed point of the first element, the center conductor connects to the second element's feed point. The other end of the line is at 50 ohms over a certain bandwidth and connected to a 50 ohm cable. The whole length of the higher Z0 cable connected to the 50 ohm cable follows the first element from its feed point to the unfed end fire of the antenna, exiting at a short placed across both elements at this end. Thus the bifilar antenna is used as an infinite balun to be able to bring a coaxial feed cable onto the antenna structure and eventually connect to its feed point. In the case of a bifilar helix antenna used as an infinite balun, the last quarter wavelength of cable before the feed point functions as a transformer that is a simple section of cable of Z0 greater than 50 ohms. For optimal matching at a center frequency, the cable characteristic impedance is calculated according to the following equation: Z0=√{square root over (ZO feed cable*Z0 antenna)}, wherein Z0 feed cable=50 ohms.
Antenna patterns in the category of bifilar antennas are of cardioid shape, with only small differences in the shape between the bifilar antenna pattern and its corresponding quadrifilar antenna pattern. As stated above, the bifilar helix antenna can be made by simply removing one of the bifilars of a quadrifilar helix antenna. Among the differences between the two designs are that the bifilar will have poorer circular polarization and pattern symmetry in the azimuth plane, since there are only two versus four elements defining a circle. Also it has more undesirable backside radiation, since the arraying of two bifilar helixes in the quadrifilar helix helps reduce backside radiation. Also the bifilar must be fed in back fire mode and must be long enough to be a traveling wave antenna before unidirectional patterns of cardioid shape occur off of the fed end of the antenna. If the bifilar is too short, then lobes will come off of both ends creating a figure eight pattern along the antenna axis. A quadrifilar helix does not have this length requirement since it is an array of two interleaved bifilars. The phasing of the array can force unidirectionality by eliminating one of the two lobes of the figure eight pattern.
If lengthening of the filar elements is necessary to maintain the cardioid shaped pattern when changing from the quadrifilar case to the bifilar case, then there will be some change in the patterns. For low pitch angles (e.g. twenty to thirty degrees) the patterns become sharper. For high pitch angles (e.g. forty to fifty degrees) patterns will split more, which may require reducing the pitch angle if acceptable overhead patterns are desired.